Question

How does pressure affect the solubility of a gas in a liquid? How does this pressure dependence account for the bubbling that occurs upon opening a can of soda?

Short answer

Pressure increases the solubility of a gas in a liquid as described by Henry's Law. Opening a soda can reduces the pressure, which decreases the gas's solubility, leading to the formation of bubbles as the gas comes out of solution.

Step-by-step solution

Understanding Henry's Law

Henry's Law states that at a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid. The formula is described as: \( S_g = k_H \times P_g \), where \( S_g \) is the solubility of the gas, \( k_H \) is Henry's Law constant for that gas in that liquid, and \( P_g \) is the partial pressure of the gas.

Relating pressure and solubility

According to Henry's Law, if the pressure of the gas above the liquid increases, the solubility of the gas in the liquid also increases. This is because the higher pressure pushes more gas molecules into the liquid.

Explaining bubbling in soda

When a can of soda is sealed, the carbon dioxide gas (CO2) above the liquid is at a higher pressure, causing more CO2 to dissolve in the soda. Upon opening the can, the pressure above the liquid drops to atmospheric pressure, so the solubility of CO2 in the soda decreases and the excess gas escapes as bubbles.

Key concepts

Solubility of Gases

Understanding the solubility of gases in liquids is fundamental in explaining various natural and industrial processes. At the most basic level, solubility refers to how well a gas can dissolve in a given liquid. Factors affecting this solubility include the nature of the gas and liquid, the temperature, and, crucially, the pressure exerted on the gas.

Solubility is a measure of concentration, usually expressed in terms of the amount of gas (in moles or grams) that can dissolve in a specific volume of liquid under particular conditions. For instance, the solubility of oxygen in water is a critical parameter in aquatic ecosystems, determining the amount of oxygen available to aquatic organisms.

In a typical scenario, a gas will dissolve in a liquid until reaching a point where the rate of dissolving gas molecules equals the rate of escaping molecules—a state known as dynamic equilibrium. Alteration in any of the factors previously mentioned can shift this equilibrium, changing the solubility.

Pressure Effects on Solubility

Pressure plays a significant role in determining the solubility of a gas in a liquid. According to Henry's Law, an increase in the pressure of the gas above a liquid, given that the temperature remains constant, results in an increase in the solubility of the gas. This effect can be visualized when considering how gases under pressure dissolve in liquids better because the increased number of gas molecules above the liquid leads to more of them entering the liquid phase.

This concept has practical implications, such as in the carbonation of beverages. Manufacturers leverage this principle by pressurizing carbon dioxide in beverages to increase its solubility, resulting in that familiar fizzy taste. On a larger scale, this also implicates how pollutants can dissolve in natural bodies of water depending on atmospheric pressures.

Gas-Liquid Equilibrium

Gas-liquid equilibrium is a state where the rate of gas molecules dissolving in the liquid is equal to the rate at which they escape back into the gas phase. At this point, even though individual molecules are in motion, the overall number of dissolved gas molecules remains constant—provided the external conditions such as temperature and pressure do not change.

This equilibrium is dynamic, rather than static, meaning that the exchanges continue but result in no net change in concentration. The concept is important for understanding processes like the aeration of ponds or the operation of pressurized gas systems. Any change that affects the gas pressure or the system’s temperature can shift the equilibrium, thereby changing the solubility of the gas.

Partial Pressure

Partial pressure is the pressure exerted by a single gas in a mixture of gases. Each gas in a mixture exerts a pressure independently, as if it were the only gas present, and the total pressure of the mixture is the sum of the partial pressures of all the gases contained within it. The concept of partial pressure is vital in the application of Henry's Law where the solubility of a particular gas in a liquid is directly proportional to its partial pressure above the liquid.

In the context of beverages, for example, the partial pressure of carbon dioxide is a critical factor in maintaining the carbonation of the drink. When the can is sealed, the partial pressure of CO₂ is high, allowing more gas to dissolve. Once the can is opened, the partial pressure rapidly decreases, leading to the formation of the bubbles as the solubility of CO₂ reduces.